Seek Overlap; Figure 71. Seek Overlap - Hitachi IC25N040ATCS04-0 - Travelstar 40GN 40GB UDMA/100 4200RPM 2MB 2.5" IDE Hard Drive Specifications

11.12 Seek Overlap

The drive provides accurate seek time measurement method. The seek command is usually used to
measure the device seek time by accumulating execution time for a number of seek commands. With
typical implementation of the seek command, this measurement must include the device and host
command overhead. To eliminate this overhead, the drive overlaps the seek command as described
below.
The first seek command completes before the actual seek operation is over. Then the device can receive
the next seek command from the host. However, the actual seek operation for the next seek command
starts right after completion of the actual seek operation for the first seek command. The execution of two
seek commands overlaps excluding the actual seek operation.
With this overlap, total elapsed time for a number of seek commands is the total accumulated time for the
actual seek operation plus one pre and post overhead. When the number of seeks is large, this overhead
can be ignored.
(1) With overlap
Host process
Device process
Seek operation
Overhead
Total time = (n-1) * (Seek operation) + A + B
(2) Without overlap
Host process
Device process
Seek operation
Overhead
Total time = n * (Seek operation + A + B)
Figure 70. Seek overlap
A
A
Hitachi Travelstar 60GH & 40GN hard disk drive specifications
B+A
97
B
B+A
B